1. Field of the Invention
The present invention relates generally to testing the endurance of flexible sheets and the adhesion of surface treatments, coatings, and print to the surface of such sheets, as well as the bonding of multilayer sheets. More specifically, the present invention relates to the creation of a small radius fold in a sheet specimen and methods of restraining the specimen with a pressure differential. The fold can be shifted progressively across the sheet in a controlled manner by manipulating the sheet specimen.
2. Description of Related Art
A number of fold endurance testers have been developed in the past. Some of these testers were capable of producing a sharp fold at one location with no sliding contact with the specimen. These devices merely folded and unfolded the sheet material repeatedly at a single line. However, a repeated fold at one location is relatively ineffective in the evaluation of printing durability or adhesion of other surface treatments. A moving line of fold or flexure is necessary for the efficient testing of sheet specimens.
There are also devices which can cause a fold line to progress and recede over the surface of a specimen by means of rubbing or rolling contact with the specimen at the line of fold. If the line of fold is created and/or controlled by a roller, the minimum radius of the fold is severely limited. Since flexural stress in a folded specimen is proportional to the inverse of the fold radius (i.e., stress can be increased significantly by decreasing the radius of the fold), a roller severely limits a flexural stress which may be applied to the specimen.
Smaller bend radii at the fold line are made possible by using a sliding contact at the inner radius on some devices. However, this approach limits the speed at which the test may be conducted, because of the heat generated by friction at the fold line. Moreover, the stress on the specimen in rubbing contact is an indefinable combination of cyclic flexure, shear at the surface, abrasion and wear. Thus, failure modes are inextricably mixed.
The present invention is for a method and apparatus for testing the flexural durability and structural integrity of sheet materials and the adhesion and durability of surface treatments (such as printing and/or coatings) which permits testing without the use of rollers or rubbing surface contact, and permits testing with bend radii approaching zero, overcomes the shortcomings of earlier devices and represents a significant advancement in the art. In those applications where flexural endurance is used to determine the adhesion of printing, coatings, or other surface treatments, rubbing and/or sliding contact of the surface against itself is unacceptable.
one immediate application of the present invention is in evaluating the print durability of currency notes. In this application, the present invention will replace a device that was intended only for testing of the structural integrity of the material being tested. It was never intended to test for the adhesion or durability of surface treatments. That device was labor intensive, time consuming and sensitive to operator manipulation. The present invention will reveal process defects not otherwise apparent by current methods.
The inventors are unaware of any method or apparatus which produces a moving, 180.degree. fold line of extremely small inside radius in a sheet specimen. No mechanically clamped specimens are required in the process. The inventors believe that the present invention is likely to fulfill a similar need within specialized segments of the paper and plastic film industries. The invention represents a breakthrough in timely durability testing of currency, removing a barrier to the development of effective quality control that has existed for many years. As a result, it is likely that the present invention has potential for widespread use. The durability testing assists also in determining the structural integrity of the materials used in producing the sheets being tested as specimens.